1. Field of the Invention
The present invention relates generally to a data reading apparatus for reading data from a recording medium such as a CD-ROM (Compact Disk-Read Only Memory) or DVD (Digital Video Disk). More particularly, this invention relates to a data demodulating apparatus which demodulates data read from a recording medium where data has been recorded in either a CLV (Constant Linear Velocity) or a ZCLV (Zone Constant Linear Velocity) system, while controlling the rotation of the recording medium at a substantially constant speed.
2. Description of the Related Art
Recently, attention has been paid to the development of optical disks to effectively record data for multimedia. Known optical disks include, for example, LDs (Laser Disks), CD-ROMs, MDs (Mini Disks) and DVDs. DVDs are about the same size as CD-ROMs, however, their capacity is about 7.5 times greater. From the conventional viewpoint of increasing the memory capacity of recording media as well as processing speeds, a data processing system would need a larger data reading apparatus that has improved data reading rates from optical disks such as a DVD. However, portable personal computers will generally require data reading apparatus that is relatively compact and which consumes less power than conventional data reading apparatuses used in desk-top computers.
A conventional data reading apparatus has a motor for rotating an optical disk and a pickup apparatus for optically reading digital data recorded on that optical disk. The pickup apparatus reads recorded data from an optical disk and outputs it as an analog signal. The data reading apparatus further has an amplifier, which amplifies an analog signal to provide an amplified analog signal having a predetermined amplitude value. The data reading apparatus compares the voltage level of the amplified analog signal with a predetermined threshold value to convert the analog signal to a binarized digital pulse signal and demodulates the digital pulse signal to produce a read data signal.
The aforementioned data reading apparatus reads data in accordance with an optical disk reading system, such as the CLV, ZCLV or CAV (Constant Angular Velocity) system (i.e., the recording system of recording data on an optical disk).
In the CLV system, data is recorded on an optical disk in the format shown in FIG. 1. Each of a plurality of tracks T arranged concentrically on the disk has a plurality of sectors each consisting of an address area A1 and a data area A2. The lengths of the individual sectors are designed to be constant regardless of the position of the tracks T which are located at inner or outer peripheries of the tracks. In reading data from such a formatted disk, the rotational speed of the disk is controlled in accordance with the position of the pickup apparatus in the radial direction of the disk. As shown in FIG. 2A, as the pickup apparatus moves inwardly on the disk from the outside, the rotational speed of the disk is increased. This change in speed permits data stored in the individual sectors to be read at the same linear velocity. FIG. 2B is a graph showing the relationship between the data rate indicative of the amount of data read by the pickup apparatus per unit time and the track position. This graph shows that at a constant linear velocity, the data rate (i.e., the frequency and amplitude of a read data signal) is constant irrespective of the position of the pickup apparatus on a disk. This is because the lengths of data recording pits in the sectors are designed to be constant with respect to the constant linear velocity, regardless of whether the pickup apparatus is positioned on the inner or outer tracks on the disk, as shown in FIG. 2C.
In the ZCLV system, data is recorded in the format shown in FIG. 3. The disk area is separated to a plurality of zones Z in the radial direction of the disk, with a plurality of tracks T assigned to each zone. The lengths of the individual sectors included in an outer track T are designed to be longer than those of the individual sectors included in an inner track T. Further, the range of the lengths of the individual sectors which vary from one track T to another in each zone Z are designed to be substantially the same zone by zone. Therefore, the sectors of each track T are radially arranged in the radial direction of the disk in each zone Z. In reading data from such a formatted disk, the rotational speed of the disk is controlled zone by zone in accordance with the position of the pickup apparatus in the radial direction of the disk.
As shown in FIG. 4A, when the pickup apparatus moves inward from the outside of the disk, the rotational speed of the disk is increased stepwise for each zone Z. This stepwise speed change permits data stored in the individual sectors to be read at the same linear velocity. As shown in FIG. 4B, at a constant linear velocity, the data rate (the frequency and amplitude of a read data signal) is approximately constant irrespective of the position of the pickup apparatus on a disk. Although the lengths of recording pits vary in each zone, the range of the change is the same for each zone, as shown in FIG. 4C. Further, the lengths of the individual sectors change in each zone may change, however, the range of the change is the same for each zone.
In the CAV system, data is recorded in the format shown in FIG. 5. The lengths of individual sectors included in the individual tracks T arranged on a disk are so designed to become longer toward the outer periphery of the disk. Accordingly, the sectors of each track T are radially arranged in the radial direction of the disk. In reading data from such a formatted disk, the rotational speed of the disk is kept constant regardless of the position of the pickup apparatus on the disk as illustrated in FIG. 6A. Further, FIG. 6B shows that at a constant rotational speed, the data rate (the frequency and amplitude of a read data signal) is approximately constant irrespective of the position of the pickup apparatus on a disk. FIG. 6C shows the lengths of recording pits in an outer sector on the disk are longer than those of recording pits in an inner sector. Therefore, the lengths of the individual sectors are also longer toward the outer periphery of the disk.
As apparent from the above-described recording systems, the CLV system that has individual sectors with lengths that are the same provides a higher disk recording density. In the case where data is recorded on disks of the same size in different systems, therefore, a disk which is recorded using the CLV system is advantageous due to its larger memory capacity.
The ZCLV system simplifies the control on the number of rotations of the disk driving motor while maintaining the advantageous features of a CLV system which has larger disk memory capacity. The ZCLV system has an intermediate disk recording density that lies between those of the CLV system and the CAV system. The CAV system typically has the lowest disk recording density of data recording systems.
Data reading apparatuses that deal with recording mediums like a CD-ROM or DVD typically employ either a CLV or a ZCLV system, which generally provides a higher recording density. Therefore, data may be recorded on a disk in accordance with the CLV or ZCLV system while the pickup apparatus reads data under the disk rotation control according to the CLV or ZCLV system.
To improve the speed at which data is read by a data reading apparatus according to the CLV or ZCLV system, the rotational speed of a disk is typically increased. Increasing the rotational speed of a disk in the CLV system will generally demand a spontaneous change in the rotational speed of the disk at the time of seeking a sector located apart from a sector on the disk in the radial direction. To meet this demand, the motor for rotating the disk should secure a sufficient driving torque. This will generally necessitate the enlargement of the motor and an increase in the power consumed by the data reading apparatus.
To remedy this problem, data may be read from a disk, which has data recorded using the CLV or ZCLV system, while rotating the disk at a constant speed under the rotational control of the CAV system. This arrangement facilitates the implementation of a motor which has a relatively small driving torque. In this manner, the physical size of the motor and the power consumption of the data reading apparatus may both be reduced. Rotating a disk at a constant speed however does not ensure a constant linear velocity. That is, the linear velocity at the time of reading data from an inner sector on a disk differs from the linear velocity at the time of reading data from an outer sector. As shown in FIG. 7, therefore, the pickup apparatus supplies the amplifier with an analog signal whose frequency F increases toward the outer periphery of the disk, and has the maximum ratio of 1:2.5. Further, as illustrated in FIG. 8, the pickup apparatus sends the amplifier an analog signal whose amplitude W decreases toward the outer periphery of the disk.
In the case where data is read from a disk, which has data recorded in the ZCLV system and under the rotational control of a CAV system, the pickup apparatus sends the amplifier an analog signal whose frequency F drops by a given level at the boundaries of the individual zones and gradually increases toward the outer periphery of the disk. The pickup apparatus also sends the amplifier an analog signal whose amplitude W increases by a given value at the boundaries of the individual zones and decreases toward the outer periphery of the disk.
It should therefore be appreciated that it is very difficult to demodulate an analog signal having a varying frequency and amplitude, and thus to accurately acquire a predetermined read data signal. Further, it is practically difficult to read data from a disk that is recorded using a CLV or ZCLV system while having disk rotation controlled by a CAV system.